The purpose of the proposed research is the study of the relationship between folic acid and vitamin B12. This will include the following specific objectives: a) The study of the metabolic relationship between folic acid, vitamin B12, and methionine, with special reference to the "methyl trap theory." b) The study of the properties of folic acid conjugate synthetase and its role in determining liver folate levels. c) The study of the relation of methionine to the metabolism of normal and tumorigenic tissue cultures. d) The effect of the carcinogen diethylnitrosamine on the metabolism of folic acid. e) The study of the availability of folic acid in foods for humans.